This invention relates to the field of turbine machinery such as jet aircraft engines and to the maintenance of close operating clearances between rotor and stator portions of such machinery.
The use of position adjustable shroud structures around the periphery of a bladed or bucketed turbine rotor to control the rotor-to-stator running clearance and thereby limit the flow of leakage gases between the rotor bucket ends and the stator has become accepted practice in aircraft jet engines and other turbo-machines. The need for this adjustable shroud usually arises from dimensional changes in the turbo machine elements--as a result of temperature changes incurred between static and operating conditions of the turbo machine. In the aircraft jet engine, for example, a significant increase in rotor diameter is often experienced between the cold engine stationary condition and the hot engine operating condition. Without the employment of movable shroud apparatus, the accommodation of this rotor dimensional change would require an undesirably large rotor-to-stator clearance when the engine is in the cold or below steady-state operating condition in order that desirable, low loss clearances be achieved during engine steady-state conditions.
The maintenance of desirable rotor-to-stator clearance in a turbo machine is, however, hampered by the occurrence of stator structure dimensional changes--also a result of temperature changes and environmental changes. Ideally, of course, the growth of rotor and stator members could be called upon to track or compensate one for the other, however, differing thermal time constants and mechanical stiffness between rotor and stator members preclude reliance on such tracking and make the addition of actuated shroud structures necessary for efficient turbo machine operation.
The maintenance of desirable rotor-to-stator dimensional clearances is therefore complicated by transient thermal responses and the attendant transient dimension changes possible during engine operation--in response to changes in engine fuel feed, engine load, or even engine environmental exposure--environmental in this sense including g forces, altitude and of course, temperature factors. Most turbo machine designs of the engine type therefore require consideration and accommodation of transient conditions in order to avoid intervals of rotor-to-stator rubbing during worst case operation.
Rotor-to-stator clearance behavior is also important in turbo machines other than the aircraft let engine--in such machines as compressors, pumps, and the steam turbines employed for generation of electrical energy and for ship propulsion, for example. In these non-internal combustion turbo machines, dimensional changes, of course, occur in response to temperature change in the gaseous or liquid fluid being processed as well as in response to environmental conditions. Usually such dimensional changes are of a smaller magnitude than those encountered in the combustion environment of an engine, but they may nevertheless be of concern because of a need for closer rotor-to-stator dimensional control in response to accuracy, efficiency or other causes.
Several arrangements of movable turbomachine shroud apparatus are disclosed in prior patents; these arrangements especially include thermally-actuated and mechanically-actuated shroud structures. Patent art examples of such movable shroud apparatus include the invention of J. S. Ingleson, shown in U.S. Pat. No. 3,085,398, wherein engine shroud elements are attached to a radially deformable support ring. The Ingleson support ring is distorted or oil-canned in the radial direction by a plurality of circumferentially engaged ramp members having engaged ramp surfaces that are displaced with respect to each other by a control mechanism in order to change the shroud radius. The Ingleson apparatus is therefore a mechanical shroud positioning arrangement but also contemplates the use of hydraulic actuators for changing the shroud cam position.
Another form of shroud actuation is shown in the patent of K. J. Albert et al, U.S. Pat. No. 3,391,904, which discloses a shroud arrangement wherein the wear strip that closely surrounds the rotating turbine blade periphery is mounted on a movable piston-like structure in order that the radius of the wear strip be responsive to the heat transfer into and out of the wear strip structure. The Albert et al invention uses bleed air from the compressor of the turbo machine to move the wear strip piston and controls this movement by way of carefully-selected heat transfer to the compressor bleed airflow. In the Albert et al apparatus the wear strip position is made an analog function of heat flow into and out of the wear strip supporting structure, in accordance with conventional practice in turbo machine shroud dimensional control. Transient conditions of engine operation can, however, lead to shroud positioning problems in the Albert apparatus: therefore the usual designer's dilemma of poor engine efficiency as a result of excessive clearance or incurring risk of rotor blade-to-wearstrip member collision during some conditions of engine operation remains.
Another arrangement for shroud-to-rotor clearance adjustment is shown in the patent of William R. Patterson, U.S. Pat. No. 3,966,354, which concerns an apparatus for supplying air selected from a varying combination of two sources, one of high temperature and the other of lower temperature, in order to achieve a desired mixture temperature and thereby the appropriate physical size for shroud mounting elements. The Patterson invention contemplates use of materials having high and low coefficients of thermal expansion and the use of a temperature-responsive cylinder member composed of the high thermal coefficient of expansion material for selecting between the different temperatured sources of air. The Patterson apparatus is especially concerned with the, rotor-to-stator clearances during transient conditions of engine operation, such as a throttle chop and a hot rotor burst.
Another rotor-to-stator or shroud clearance control arrangement is shown in the patent of Claude Christian Hallinger et al. U.S. Pat. No. 3,975,901. The Hallinger patent also discloses an apparatus providing the selection of stator dimension controlling air from sources supplying air at two different temperatures. The Hallinger et al invention employs an air control valve called an obturator that is responsive to the temperature of the fluid passing through the turbine for selecting relative quantities of hot and cold air for mixing. The mixed air is applied to the wall of the turbine stator to achieve stator dimension control and rotor-to-stator clearance control. The Hallinger et al invention employs the thermal expansion of an otherwise rigidly-mounted stator member for achieving the rotor-to-stator clearance control.
Yet another arrangement for turbo machine rotor-to-stator clearance control is shown in the patent of John Jenkinson, U.S. Pat. No. 3,982,850, and involves stator element positioning through thermal expansion of insulation-surrounded stator support components. The insulation used for the stator support components in the Jenkinson invention is comprised of a metallic sheet which is dimpled and spot-welded to the dimension control elements at predetermined selected intervals in order to provide a desired degree of heat conduction between insulation and the temperature-controlled component. The Jenkinson invention is based on the concept, therefore, of determining seal clearances by matched thermal expansion of seal-carrying components during varying conditions of turbo machine operation. The number and sizes of the areas of attachment between insulating sheet and the expansion control member are selected to define the required heat-conducting path into the expansion control element of the Jenkinson invention.
Yet another clearance control arrangement and a discussion of several additional of such arrangements is shown in the patent of Glen W. Thebert, U.S. Pat. No. 4,247,247 which also concerns a fluid pressure operated clearance control apparatus. The Thebert apparatus achieves blade tip clearance control by deflecting or deforming a portion of the engine housing using several sequential pressure chambers. Temperature sensing and tachometer sensing of engine operating conditions is also employed in the Thebert invention.
Although each of these clearance control arrangements is a useful advancement of the turbo machine art, none has achieved a fully satisfactory arrangement for controlling rotor-to-stator clearances under the varying conditions of operation encountered by such equipment. Most of these arrangements fail to provide the desired minimum clearance because it is difficult to exactly match the rotor time constant with the stator time constant--even with active or passive thermal methods. The present invention provides extremely fast actuation time relative to the rotor and stator thermal response time, and thereby solves this problem.